This invention relates to a semiconductor device with an improved surface protecting structure.
A planar type semiconductor device operable at a high voltages is widely used in many kinds of the electric products. It is well known that the maximation operation voltage of a planar type semiconductor device is influenced by the condition of the surface where the p-n junction is exposed and the neighborhood thereof. A mesa shaped semiconductor device has a high operating voltage, more than the planar type under the same doping conditions, but the mesa shaped device is difficult to manufacture. Therefore, some attempts have been made to increase the operating voltage of the planar type device including guard ring structures which extend the area of a depletion region produced by applying a reverse bias voltage to the p-n junction. The electric field at the surface is divided and depresses the inner portion of the p-n junction.
Glass passivation is also effective as a surface protecting method. The glass passivation film covers the exposed portion of the p-n junction and the outer surface including the guard ring region surrounding the p-n junction. Conventional planar type transistors having a high operating voltage have used the glass passivation film on the surface region which extends from the base junction to the area which surrounds the guard ring region, but this structure has some defects. One is the glass film is damaged by cracking and breaking during manufacture because the glass film of the above-mentioned conventional device has a large area. Glass is easily broken so that the probability of damages is high. The glass has a different coefficient of thermal expansion from the semiconductor substrate so that damage results.
Another concern is low current gain. The active area of the transistor is narrow in consequence of the need to form the guard ring region beside the active area. In addition, the glass film formed on the base p-n junction degrades the accuracy of fine photo engraving process. Because the glass film is thicker than 10 microns, the resolution decreases near the edge of the glass film. Therefore, the emitter region has to keep apart more then 100 microns from the edge of the glass film. The exact photo engraving process does not accomplish and the actual operable area of the transistor is too narrow.